Chantho was portrayed by Chipo Chung.
100 trillion years in the future, the universe was nearing its end as stars burnt out and whole galaxies collapsed. Chantho found herself the last of her race, the Malmooth. When humans arrived on her home planet of Malcassairo, she befriended Professor Yana and became his assistant, working with him to build a rocket that would take the last of mankind to Utopia, a fabled colony or city existing somewhere out in the darkness.
When the Doctor, Martha Jones and Jack Harkness arrived in the silo, they helped Yana and Chantho with the final stages of the plan and succeeded in launching the rocket. During this time, however, Martha discovered that Yana carried a fob watch similar to the one that the Doctor used to turn himself human in the episode "Human Nature". The watch had Gallifreyan symbols etched upon it and Yana looked at it like he could barely see it due to its perception filter. Yana opened the watch and released the essense of the man he truly was: the renegade Time Lord known as The Master.
When the Master awoke, he sealed the Doctor and his friends inside the silo complex but allowed the cannibalistic Futurekind to enter. Chantho tried to stop him and pointed a laser gun at him, but found herself unable to shoot her former friend as he advanced on her, brandishing an electrical cable. The Master electrocuted Chantho and left her for dead as he prepared to steal the TARDIS, but Chantho held on to life long enough to raise her blaster and shoot him. She then collapsed and died.